The present invention relates generally to wire-feed welding devices and to methods and apparatus for controlling such wire-feed welding devices.
A common metal welding technique employs the heat generated by electrical arcing to transition a workpiece to a molten state, followed by addition of metal from a wire or electrode. One technique that employs this arcing principle is wire-feed welding. At its essence, wire-feed welding involves routing welding current from a power source into an electrode that is brought into close proximity with the workpiece. When the electrode contacts the work piece, current flows, and an arc is established from the electrode to the workpiece, completing a circuit and generating sufficient heat to melt and weld the workpiece. Often, the electrode is consumed and becomes part of the weld itself. Thus, new wire electrode is advanced, continuously replacing the consumed electrode and maintaining the welding arc. If the welding device is properly adjusted, the wire-feed advancement and arcing cycle progresses smoothly, providing a good weld. One common type of wire-feed welding is metal inert gas or “MIG” welding.
Traditionally, operating power for generation of the electrical arc is produced by rectifying and conditioning an AC power signal. To accomplish this, traditional MIG welding devices employ silicon controlled rectifiers (SCRs) to condition and convert incoming AC power (i.e., from the power grid) into an appropriate output power. In general, the SCRs are fed an AC waveform and are switched from a non-conducting state to a conducting state at particular points along the waveform to supply voltage at a desired level to downstream circuitry, particularly to a capacitor that is charged and that ultimately supplies a DC waveform to the welding torch. As will be appreciated by those of ordinary skill in the art, the point along each half-cycle lobe of the AC waveform at which the SCRs are switched to their conductive state is commonly referred to as the “firing angle.” The firing angle is often expressed as a time interval, e.g., the number of milliseconds from the current zero-crossing (i.e., the trailing zero-crossing of the half-cycle lobe) of the given AC wave form to “firing” (switching to the conducting state) of the SCR. Generally speaking, the earlier in the cycle the firing angle occurs, the greater the average output voltage through the SCRs, assuming the SCR is switched to its conducting state after the peak of the lobe.
In traditional welding systems, control of the SCR firing angle is effectuated by rigid adherence to a closed-loop, feedback control scheme, in which the firing angle varies based on the voltage output feedback returned to the controller. That is, in traditional systems, the SCR firing angle takes into account both a command or desired output voltage signal and a feedback voltage during operation, the system then constantly comparing the two and attempting to maintain the set or desired voltage. Unfortunately, the voltage feedback values of the system will vary relatively dramatically, particularly during start-up, or initiation of a weld, sometimes referred to as “arc initiation”. In fact, the voltage feedback value will vary from an open circuit voltage (because no arc has been struck and no current flows through the workpiece and back to the power supply), to a short circuit voltage value once the wire electrode comes into contact with the workpiece and the arc is initiated. This variance may result in extinguishing of the welding arc or delay in establishment of the arc, or generally erratic operation, as the controller attempts to manage transitions between what it interprets as open and short circuit conditions. Moreover, this variance can lead to increased weld spatter during start-up, flaring, stumbling, torch pushback, among other problems, all of which are undesirable.
Therefore, there exists a need for improved apparatus and methods for the control of wire-feed welding devices.